high computer slow render blender
It is normal and logical that Blender use all available cores and CPU time for render. You have two options for change it. 


You can set low priority in system task manager (how to do it depend on system you are using, this is about system, not about Blender)
You can set number of used cores for render.


EDIT: actually point one does not change percentage of usage, only set low priority, so your CPU will work on 100%, but will preferentially prioritize other running programs ans services